During the recent ZGC + Loom rewrites (JDK-8296875) a bug was introduced, which causes Loom to thaw stackChunks with stale oops.

Before that change we made sure that only the GC performed load barriers on the cont oop in stackChunk, so that we could use the oop state as an indication of the age of the stackChunk. That information was then used in ZCollectedHeap::requires_barriers.

In Generational ZGC we changed the implementation of requires_barriers, so that the rest of the JVM didn't need to have special-handling of the cont oop. However, when upstreaming the changes in JDK-8296875 we purposefully didn't update requires_barriers, but we incorrectly brought over the removal of the special-handling of the cont oop.

To limit the fix for this bug, I propose that we restore the special-handling of the cont oop.